[SPARK-3399][PySpark] Test for PySpark should ignore HADOOP_CONF_DIR and YARN_CONF_DIR

Author: Kousuke Saruta <sarutak@oss.nttdata.co.jp>

Closes #2270 from sarutak/SPARK-3399 and squashes the following commits:

7613be6 [Kousuke Saruta] Modified pyspark script to ignore environment variables YARN_CONF_DIR and HADOOP_CONF_DIR while testing

bin/pyspark

@@ -85,6 +85,8 @@ export PYSPARK_SUBMIT_ARGS
 
 # For pyspark tests
 if [[ -n "$SPARK_TESTING" ]]; then
+  unset YARN_CONF_DIR
+  unset HADOOP_CONF_DIR
   if [[ -n "$PYSPARK_DOC_TEST" ]]; then
     exec "$PYSPARK_PYTHON" -m doctest $1
   else